Up to now, a polyvinyl alcohol-based resin (polyvinyl alcohol is referred to as PVA hereinafter) has been used suitably as a dispersant in the emulsion polymerization of ethylenically unsaturated monomers.
Among such ethylenically unsaturated monomers, vinyl acetate has high affinity for PVA-based resins and can produce an aqueous emulsion having excellent stability, but since the thus obtained vinyl acetate resin emulsion has high minimum film-forming temperature, there is a problem in that film-forming property and adhesiveness become insufficient under a low temperature environment.
On the other hand, since an acrylic-based emulsion has sufficient film-forming property even under a low temperature environment, there is increasing a case in which such an acrylic-based resin emulsion is applied to applications wherein a vinyl acetate resin emulsion has been conventionally used.
However, in the case of the acrylic-based resin emulsion, emulsification dispersion property is insufficient by a general unmodified PVA-based resin and, what is more, acrylic-based monomers have smaller radical reactivity in comparison with vinyl acetate, so that there is a case in which initial reaction is difficult to be generated in the presence of a PVA-based resin or stability at the time of polymerization becomes insufficient.
In order to solve such problems in the production of acrylic-based resin emulsions, examinations are broadly carried out on the use of various modified PVA resins having further high dispersion stability, as the PVA-based resin.
For example, as a dispersant which can provide an acrylic-based resin emulsion having excellent stability at the time of emulsion polymerization and having excellent long-term standing stability, dilute stability and mechanical stability, a PVA-based resin comprising a polyoxy alkylene group in its side chain has been proposed (e.g., see Patent Reference 1).
In addition, a dispersant consisting of such a polyoxy alkylene group-containing PVA-based resin is suitably used in a polymerization method by a pre-emulsion method in which an acrylic-based monomer is emulsified and dispersed in water in the presence of a dispersant and the thus prepared pre-emulsion is polymerized by adding it dropwise to a polymerization reaction system.
In this connection, the polyoxy alkylene group-containing PVA-based resin used in the Patent Reference 1 has a degree of saponification of from 30 to 80% by mol, and those having a degree of saponification of from 35 to 73% by mol, namely those having a region of lower degree of saponification as a PVA-based resin, are used in the Examples.
In addition, as a dispersant which can provide an acrylic-based resin emulsion having excellent mechanical stability and freezing stability and also in long-term standing stability at high temperature, a PVA-based resin comprising a 1,2-diol structure in its side chain has been proposed (e.g., see Patent Reference 2).
In this connection, there is described in this Patent Reference 2 that those which have a residual ester group quantity of 15% by mol or less (those having a degree of saponification of exceeding 85% by mol) are desirable as the PVA-based resin comprising a 1,2-diol structural unit, and in the Examples, those having a residual ester group quantity of from 2.2 to 6.0% by mol (from 94.0 to 97.8% by mol as the degree of saponification), namely those having a high range of the degree of saponification, are used as the PVA-based resin.    Patent Reference 1: JP-A-2000-297107    Patent Reference 2: JP-A-2006-124682